ES2581234T3 - Low NOx burner - Google Patents

Abstract

Oven with low NOx emissions (2) comprising: - a wall (4) and a combustion chamber (8) inside the wall (4), - a low NOx burner (10) adapted for use recirculation of oven gas inside the combustion chamber (8) of the oven (2), comprising the burner (10): - an elongated tube (24) for connection to a combustion air supply (90), - a combustion air centrifuge (36) defining a burner shaft (10), - a plurality of first fuel gas spindles (40) having fuel gas discharge orifices (46), in which, - the tube (24) is installed in the wall (4) and extends a substantial distance from the wall (4) into the combustion chamber (8), - a plurality of elongated air holes (50) for connection to the combustion air supply (90) and extending from the wall (4) into the combustion chamber (8), downstream of the discharge ends of the air holes (50) that are separated from the oven wall (4) and the centrifuge (36), - the fuel gas discharge holes (46) of the first fuel gas spigots ( 40) are in the vicinity of one end downstream of the centrifuge (36), - a second fuel gas tap (66) disposed between each adjacent pair of air holes (50), is connected to a source of combustible gas (12 ) and has fuel discharge holes (68) downstream of the oven wall (4) and upstream of the discharge ends, characterized in that: - the combustion air centrifuge (36) is connected to the tube (24) so that one end downstream of the centrifuge (36) is inside the combustion chamber (8) and away from the furnace wall (4), - said second fuel gas tap (66) is arranged in relation to the axis near the radially outermost parts of the air holes (fifty).

Description

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DESCRIPTION

Low NOx burner Background of the invention

The present invention relates to burners with low NOx emissions that are compact, effective at the time of operation and employ recirculation of furnace gas inside the combustion chamber of the oven to reduce NOx emissions.

Furnace emissions are a matter of great concern since they contribute significantly to air pollution. A large source of NOx emissions is burners as used in large and small furnaces, including, for example, very large furnaces used to generate electric power with steam-operated turbines. It is well known that NOx emissions are reduced by lowering the temperature of the flame generated by the burner inside the oven. Conventionally, this has been achieved by supplying the burner with excess air with respect to what is required to stoichiometrically burn the fuel, since the fuel must heat the additional air, which lowers the overall temperature of the flame and the oven gases generated by it.

Another approach to lower NOx emissions is to mix the combustion air for the burner with chimney gas that goes to the exhaust pipe. This technique is called chimney gas recirculation (FGR). Chimney gas normally has a temperature in the range of approximately 93 ° C (200 ° F) to 204 ° C (400 ° F). The recirculated chimney gas lowers the flame temperatures and the generation of NOx, but in excessive quantities it causes instability of flame and explosion.

Both approaches can be used individually or in combination. However, the large amounts of FGR that may be needed to reduce NOx substantially increase the volume of global gas to be transported through the burner and the oven convection section. This in turn requires larger blowers and ducts, including the common wind box outside the front wall of a burner, to handle the combined increased mass of air and FGR with an elevated temperature that must be transported through the system. This increases the initial installation costs as well as the subsequent maintenance and operation costs due to the increased energy requirements of the blower, all of which is not desired.

US 2005/0271990 A1, which discloses an oven according to the preamble according to claim 1, discloses a burner assembly that produces very low NOx emissions. The large amounts of FGR that must be recycled can be reduced by recirculating furnace gases internally from the combustion chamber. This has worked well in reducing NOx emissions and has the advantage that it reduces or eliminates additional energy to operate a larger blower to handle combustion air and / or additional recirculated chimney gas. The main part of the burner is a solid cylindrical tube that extends from the oven wall. The centrifuge is mounted on the discharge end of this tube. The part of the tube next to the furnace wall includes openings through which furnace gases are conducted aerodynamically by air and jets of fuel gases inside the tube where the furnace gases are mixed with combustion air and the fuel before switching on the mixture. However, this burner is prone to overheating and damaging the tube if the fuel begins to burn within the limits of the tube. Conditions may exist for the fuel that burns inside the tube when the overall incoming mixture of air, chimney gas and combustible gas is insufficiently diluted with inert gases such as FGR. Separating the operating regimes of the burner from the flame that burns inside also requires additional displacement towards the discharge end of the tube which is usually not optimal to achieve lower NOx emissions.

US 5,460,512 describes a furnace with low NOx emissions according to the preamble according to claim 1.

US 6,685,462 B2 discloses an apparatus for burning fuel with low NOx formation. A burner housing, attached to an oven, is provided which has means for mixing a first part of the fuel gas with a first part of the air to form a primary mixture of primary fuel-air gas and to discharge the primary mixture of combustible gas- air into a primary burn zone in the oven from at least one discharge location surrounded by a wall extending into the oven. The outer sides of the wall part of the housing are inclined towards the central area of the base part.

Brief summary of the invention

The present invention further improves the low NOx burner described above because it eliminates the need for a tube that encloses the burner and simplifies the construction and operation of the burner as described below.

A burner with low NOx emissions constructed according to the present invention is installed in an oven having

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an oven wall that encloses the combustion chamber of the oven. The burner is installed in a wall of the oven and extends through an opening in it to the interior of the combustion chamber, where it generates a flame.

The burner itself has a combustion air centrifuge that is completely arranged in the combustion chamber, and its downstream end is separated a substantial distance from the furnace wall, as described further below. A combustion air tube extends into the combustion chamber, supports the centrifuge and causes combustion air to flow from a combustion air source to the outside of the oven through the centrifuge into the combustion chamber.

A plurality of air holes, preferably six, may be used, but they may be more or less, extending from the furnace wall into the combustion chamber. They are circumferentially equidistant from each other to define spaces between them and to normally supply an important part of the required combustion air alone or, when necessary, mixed with FGR. Its discharge ends are arranged inside the combustion chamber, upstream of the centrifuge, and are separated from the centrifuge and the oven wall.

Suitable plates between adjacent air holes block combustion air from flowing from the combustion air source into the oven except through the holes and the pipe in the center of the burner.

A first set of elongated fuel taps, preferably a number of fuel taps that corresponds to the number of air holes, extends from the fuel source beyond the kiln wall into the combustion chamber. Its fuel gas discharge holes are separated at the ends of the furnace wall spikes at least as much as the downstream end of the centrifuge so that fuel gas is discharged into the combustion chamber, where the fuel gas It is mixed with combustion air from the centrifuge.

At least a second fuel tap is located in each cavity space between adjacent air holes, and extends from the fuel source beyond the kiln wall into the combustion chamber. Each second fuel gas tap is radially separated from the burner shaft so that it is located next to a radially outermost part of the adjacent holes. Each second fuel tap has a downstream end that includes one or more fuel discharge holes arranged inside the combustion chamber and inside the cavities, downstream of the kiln wall and upstream of the ends of discharge of the air holes.

The aerodynamic forces created by the second jets of fuel and the flow of air that is discharged through the air holes cause a circulation of combustion products (hereinafter also referred to as "furnace gas") from the flame in the combustion chamber back to the front wall of the oven. During this circulation, the combustion products are partially cooled due to the transfer of heat to the walls of the furnace water pipe. As a result, the combustible gas that propagates from the second spigots through the space between the air holes is first mixed with essentially inert reduced temperature furnace gas. This non-combustible mixture is further mixed with combustion air from the discharge ends of the air holes upstream of the centrifuge for subsequent ignition of the mixture by means of the flame in the combustion chamber on the downstream side of the centrifuge.

The burner is also preferably associated with a fuel gas valve or regulator that is operatively coupled with the fuel gas source and is set to direct relatively more fuel gas through the second fuel gas spindles than through the first fuel gas spindles.

According to a presently preferred embodiment of the invention, the burner includes a third set of fuel gas spouts with nozzles that are disposed inside the respective air holes. The third fuel gas nozzles are located along the central lines of the air holes (usually multiple nozzles are arranged in each air hole, for example, along the radial center line of the air hole). The size and location of the nozzles are chosen to create an approximately uniform fuel distribution with the air flow. All of the third nozzles inject the fuel in the same direction as the surrounding air currents.

The above-mentioned cavities between adjacent air holes are circumferentially open inside the combustion chamber, and neither the air tube nor the centrifuge is enclosed inside a tube or conduit so that they are in the gas recirculation of oven. This means that the furnace gases that recirculate inside the combustion chamber can enter the cavities between adjacent air holes, where they are mixed with combustible gas to form a non-combustible mixture of fuel gas / flowing furnace gas. in a downstream direction towards the centrifuge. Downstream of the air orifice, this mixture is further mixed with combustion air from the air orifices and forms a mixture of combustible gas / combustion air / furnace gas that can be ignited by the existing flame downstream of the centrifuge.

For specific applications, a mixture of combustion air and FGR may be desired, or needed, to supply the wind box. This alternative is preferably limited to applications where they must be obtained.

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particularly low NOx emissions, below those that can be achieved only with furnace gas recirculation, since it requires blowers, pipes, wind boxes, etc. older and therefore more expensive.

In the operation following the initial ignition of the burner, the flame generated by the burner is anchored at the downstream end of the centrifuge, relatively far from the front wall of the oven where the burner is mounted. Since the burner is not enclosed inside a tube or tubular element and the main air discharge holes are located relatively close to the front wall of the oven, although the centrifuge is relatively far from the wall and far inside of the combustion chamber, the flow rates of the combustible gas, the combustion air and its mixture have decreased significantly by the time they reach the centrifuge. This avoids the problem encountered with the typical prior art burners that are located inside and near the ends of surrounding tubular ducts where the velocities of the mixture of higher combustible gas and combustion air can lead to flame instabilities and relatively early flame extinctions when trying to achieve the lowest NOx emissions. With the burner of the present invention, the air and gases discharged are not restricted to limited cross sections and, therefore, decelerate relatively quickly, which aids in the stabilization of the flame in the centrifuge. Therefore, the present invention lowers the flow rate of the gases surrounding the centrifuge, increases flame stability and significantly lowers the possibility of flame extinctions, while lower NOx emissions are achieved with a burner that is less expensive to build, install, maintain and operate than comparable prior art burners.

In addition, by placing all the fuel gas taps inside the radially outermost extension of the air holes and eliminating a burner throat traditionally formed by the oven wall, the radial burner cover (in relation to the oven wall ) is reduced so that it takes up less space on the front wall of the burner and inside the oven chamber. This feature is particularly advantageous for retrofitting existing furnaces with low NOx burners where the size of the opening available for the burner is limited by the front wall water pipes (since the low NOx burners currently available are normally significantly larger than conventional burners due to their need for higher FGR rates and additional features needed to lower NOx).

Brief description of the drawings

Figure 1 is a cross-sectional view in schematic side elevation of a burner with low NOx emissions made according to the present invention, installed on a furnace wall and taken in the I-I line of Figure 2.

Figure 2 is a front elevation view of the burner shown in Figure 1.

Figure 3 is a schematic diagram illustrating the recirculation of furnace gases inside the combustion chamber of the oven according to the present invention.

Detailed description of the invention

With reference to the drawings, an oven 2 has a front wall 4 with an opening 6 that provides access to the inside of a combustion chamber 8 inside the oven. A burner with low NOx emissions 10 constructed according to the present invention extends through the opening 6 into the furnace combustion chamber 2, where it forms a flame 84 to generate heat. For example, the oven can be a boiler that generates steam.

A fuel gas supply 12 and a combustion air supply 90 are suitably coupled to the wind box 14 attached to the front wall 4 of the oven. The burner directs the fuel and combustion air into the combustion chamber, where they mix, ignite and burn, thereby releasing thermal energy and generating high temperature furnace gases that are normally discharged inwards. of a convection section 16 of the furnace where the temperature is reduced, usually up to a range between about 93-204 ° C (200-400 ° F). The cooled chimney gas is discharged into the atmosphere through an exhaust pipe 20. As will be explained in detail below, a part of the cooled chimney gas is sometimes recirculated into the combustion chamber by means of a chimney gas recirculation system 18.

Referring now specifically to Figures 1 and 2, the burner 10 has an elongated burner shaft 22 which is also the axis of a combustion air tube 24 which is supported by a suitable tube support 26 on a plate 28. A upstream or rear end 30 of the tube is open, extending into the interior of the wind box 14, and has a damper 32 that can be used to adjust the flow of combustion air into the tube, as is well known to experts usual in the art.

At its downstream end 34, the burner tube supports a combustion air centrifuge 36 having a downstream end with the centrifuge blades 38. The combustion air tube is long enough so that the downstream end of the centrifuge is located at a substantial distance from the front wall 4 of the

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oven. In one embodiment of the invention, the burner tube has a diameter of approximately 16.5 cm (6.5 inches) and the downstream end of the centrifuge is separated from the oven wall approximately 112 cm (44 inches), so that the downstream end of the centrifuge is separated from the oven wall slightly less than six times the diameter of the tube. For most applications, the distance between the front wall of the oven and the downstream end of the centrifuge will be in the range of approximately four to eight times the diameter of the combustion air tube 24, although for particular purposes and installations and oven settings this interval may be greater or less.

In the illustrated embodiment, a plurality of six central fuel gas spouts 40 are circumferentially equidistant apart around the periphery of the centrifuge 36, held in place in the centrifuge by suitable spigot braces 42, and their downstream ends 44 they are separated from the oven wall 4 at least as much as the downstream end 38 of the centrifuge and, preferably, extend slightly beyond the centrifuge, as illustrated in Figure 1. The downstream ends of the central spindles have holes 46 from which fuel gas is discharged into the eddy air flow that passes through the centrifuge. An upstream end 48 of each central spigot is fluidly coupled to the fuel gas source 12, shown in Figure 1 as a circular fuel gas supply tube or manifold 12a.

In the illustrated embodiment, a plurality of six combustion air orifices 50 formed by elongated ducts are circumferentially equidistant around the combustion air tube 24, as best seen in Figure 2. Each air orifice it is formed by radially internal and external walls 54, 56 and side walls 52. The cross section of the air holes has a decreasing section in a downstream direction by the side walls 52 so that an upstream end 58 of the air hole it has a larger cross section than a discharge end downstream 60 thereof. The discharge end in turn has a decreasing section (as can be seen in the best way in Figure 1) so that the outermost wall 56 of the air hole extends further into the combustion chamber 8 than the innermost wall 54 thereof. This decreasing section induces a deviation in the combustion air that flows through the air orifices that directs the air flow to the centrifuge 36 for ignition by means of the flame on the downstream side of the centrifuge.

For typical burner constructions according to the present invention, the separation between the front wall 4 of the oven and the discharge end 60 of the air orifices 50 is in the range of about a quarter to half the distance between the wall of oven and the downstream end 38 of centrifuge 36. In a particularly preferred embodiment of the invention, the discharge end of the air hole is 16 inches apart from the oven wall, while the downstream end of the centrifuge is 112 cm apart. (44 inches). However, these intervals can be exceeded above or below if desired for a given installation.

Between each adjacent pair of air holes there is an open radially outward gap that is closed in an upstream direction by the burner plate 28 and the thermal insulation 62. The spaces between adjacent air holes form cavities 64 that are closed in a rear direction and also substantially in a radially inward direction and which are open in the downstream and radially outward directions, as can be seen in Figure 1. As a result, combustion air does not flow effectively from the windbox 14 inwards through the cavities.

The center pins 40 extend through the burner plate 28 inwards and beyond the cavities 64 towards the centrifuge in the combustion chamber. An additional set of second fuel gas spikes 66 is disposed near the radially outermost part of cavities 64 that is proximate to outer walls 56 of the air holes 50. The downstream ends of the second spindles have holes 68. The ends downstream of the second pins 66 with holes 68 are located in the combustion chamber just downstream of the kiln wall 4 and upstream of the discharge ends 60 of the air holes 50 in the cavities 64. The upstream ends 70 of the pins 66 are fluidly connected to the fuel source 12 in the form of a second circular fuel gas manifold 12b. The combustible gas that flows through the holes 68 flows into the cavities 64.

A third set of fuel spikes 72 is preferably disposed within each air orifice 50 and includes an elongated nozzle tube 74 that extends transversely with respect to the flow direction, preferably along the center line of the hole of air, through the air hole and has fuel gas discharge holes 76. An upstream end 78 of the third set of spigots 72 is fluidly connected to the fuel gas supply 12 in the form of a third fuel gas manifold circular 12c. Each tap 72 normally has multiple discharge holes 78 that are located along the central lines of the air hole. The size and location of the nozzles are chosen to create an approximately uniform distribution of fuel in the air stream. The holes 76 have central lines that are oriented in the direction of the axis 22 as shown in Figure 1.

During use, combustion air flows from the wind box 14 through the air holes 50 beyond the discharge ends 60 thereof in a downstream direction as described above. The

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Gas discharge nozzle tubes 74 in the air holes have a detrimental resistance to combustion air flow that is proportional to the second air velocity power around the nozzle tubes 74. To minimize this resistance, the tubes 74 they are located inside the holes 64 in a location where the cross section of the air holes (in the plane perpendicular to the axis 22) is substantially larger than the cross section of the air hole at the discharge end 60 so that The air flow rate beyond the nozzle tubes 74 is substantially less than its velocity at the discharge end.

A pilot 80 shown in Figure 1 is properly located inside at least one of the air holes 50 and is activated to initially ignite a first part of a combustion-fuel gas mixture formed downstream of the tube of fuel gas nozzle 74. The flame originated by the pilot further extends beyond the discharge end of centrifuge 38, where it ignites the rest of the fuel supplied to the burner.

A fuel gas flow regulator 82 receives fuel gas from source 12, directs controlled amounts of the fuel gas to the fuel gas manifolds 12a-c and controls the amount of fuel gas supplied to each of the manifolds. For normal typical operations of the furnace gas, the fuel gas regulator supplies between approximately 5 and 20% of the total fuel gas requirements to the central spouts 40, between approximately 30 and 70% of the total gas requirements to the external spigots 66, and between about 10 and 40% of the fuel gas requirements to the fuel gas spikes 72 inside the air holes 50.

To start the furnace, the burner 10 is initially activated by injecting air from the wind box 14 into and through the combustion chamber 8 of the oven to purge the combustion chamber of any fuel residue that may be Present. To ignite the burner, a reduced combustion air flow is initiated through the air tube 24 and the air holes 50 into the combustion chamber. The pilot light 80 is illuminated in at least one air hole 50 to generate a flame that extends forward towards the centrifuge 36, and the fuel gas flow regulator 82 is opened to flow fuel gas beyond the holes at the downstream ends of the internal pins 40, the external pins 66 and the pins 72 inside the air holes 50. Thus, the pilot flame and the ignited combustible gas extend beyond the downstream end 38 of centrifuge 36, which causes the ignition of the combustible gas emitted by all the fuel gas spindles of the burner.

Once a flame is lit downstream of the centrifuge 36, the pilot 80 goes out. The flame extending from inside the air holes 50 to the centrifuge is extinguished due to the lack of flame stability inside the air holes without the presence of a sufficiently strong pilot flame. The operation of the burner continues with a flame 84 formed inside the combustion chamber 8 and downstream of centrifuge 36, fueled by the burner spigots and combustion air discharged into the combustion chamber by means of the combustion chamber. centrifuge 36 and air holes 50.

The impulse of air and jets of fuel coming out of the discharge ends of the holes 50 and the impulse of jets of combustible gas from the holes 68 in the cavities 64 cause a recirculation 86 of furnace gases from internal parts of the chamber of combustion (downstream of centrifuge 36) towards the front wall 4 of the oven, as illustrated in Figure 3. The recirculation furnace gases are usually partially cooled from the initial flame temperature by heat transfer to the oven walls covered with tubes 88 normally arranged inside the oven, for example along the walls thereof. Some of the recirculating chimney gas is introduced into the cavities 64 between adjacent pairs of air holes 50 where combustible gas is drawn from the external spigots 66 in the furnace gas. Downstream of the discharge ends 60 of the air orifice, this fuel / furnace gas mixture is mixed with combustion air from the air orifices 50, which typically includes fuel gas from the nozzle tubes 74 of the third set of tap 72. The combustion gas / combustion air / fuel gas mixture flows to centrifuge 36 as described above, and downstream of centrifuge 36 the mixture is ignited by flame 84 stabilized by the action of centrifuge 38.

The entrainment of recirculation furnace gas into the fuel gas / combustion air mixture results in a reduced flame temperature 84, which at the same time reduces the generation and emission of NOx. This is advantageously obtained without an increase in the inward flow of and through the oven convection section 16 and without the need for a larger blower 92 and duct sizes that are required if the flame temperature is reduced , for example, increasing the flow of recirculation of chimney gas 18.

Also, when the recirculation furnace gas again reaches the front of the boiler, it normally has a temperature of approximately 538 to 1093 ° C (1000 to 2000 ° F). When this gas is mixed with flows from air holes 60, the overall temperature of the resulting mixture increases before it is ignited to approximately 316 to 427 ° C (600 to 800 ° F). This substantially increases the relationship between gas temperatures before and after ignition (for a flame with very low NOx emissions, its temperature is approximately 1371 ° C (2500 ° F)). As a result, the combustion process is initiated and maintained more easily. This stabilizes the flame and constitutes a significant benefit obtained with the present invention.

If it is necessary to reduce NOx emissions to below what is feasible by means of recirculation furnace gas inside the combustion chamber, some of the chimney gas is added to the combustion air by means of a recirculation system chimney gas 18. The recirculated chimney gas lowers the available oxygen supply in the fuel gas / combustion air / recirculated oven gas mixture, which leads to an additional reduction in flame temperatures and thereby the content NOx of the furnace gas before it is discharged to the environment by treating chimney gas 16 and the exhaust pipe 20.

The described device makes it possible to achieve lower NOx emissions with a stable flame than other known devices that occupy the same global space on the front wall of the oven, and is generally more energy efficient to supply comparable levels of NOx emissions.

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Claims (12)

1. Oven with low NOx emissions (2) comprising: - a wall (4) and a combustion chamber (8) inside the wall (4), - a burner with low NOx emissions (10) adapted to use oven gas recirculation in 5 the inside of the combustion chamber (8) of the oven (2), the burner (10) comprising: - an elongated tube (24) for connection to a combustion air supply (90), - a combustion air centrifuge (36) defining a burner shaft (10), - a plurality of first fuel gas taps (40) having fuel gas discharge holes (46), 10 in which, - the tube (24) is installed in the wall (4) and extends a substantial distance from the wall (4) into the combustion chamber (8), - a plurality of elongated air holes (50) for connection to the combustion air supply (90) and extending from the wall (4) into the combustion chamber (8), downstream of the 15 discharge ends of the air holes (50) that are separated from the oven wall (4) and the centrifuge (36), - the fuel gas discharge holes (46) of the first fuel gas taps (40) are in the vicinity of a downstream end of the centrifuge (36), - a second fuel gas tap (66) disposed between each adjacent pair of air holes (50), 20 is connected to a source of combustible gas (12) and has fuel discharge holes (68) downstream of the oven wall (4) and upstream of the discharge ends, characterized in that: - the combustion air centrifuge (36) is connected to the tube (24) so that one end downstream of the centrifuge (36) is inside the combustion chamber (8) and away from the oven wall 25 (4), - said second fuel gas tap (66) is arranged in relation to the axis close to the radially outermost parts of the air holes (50). 2. Oven with low NOx emissions (2) according to claim 1, which includes a third fuel tap (72) disposed inside each air hole (50) and having a discharge hole of 30 combustible gas (76) located upstream of the discharge end to inject combustible gas into air of combustion flowing through the air hole (50). 3. Oven with low NOx emissions (2) according to claim 1, wherein each air orifice (50) forms an elongated duct having a cross section that is the largest at one end upstream of the duct and the most small at one end downstream of it so that, after flowing air from 35 combustion through the duct, the combustion air velocity is the highest at the end of duct discharge. 4. Oven with low NOx emissions (2) according to claim 3, which includes a third fuel gas tap (72) disposed in each duct, and wherein the third fuel gas tap (72) is placed inside of the conduit at a location upstream of the discharge end of the conduit 40 where the velocity of combustion air beyond the third spikes of combustible gas (72) is lower than the combustion air velocity at the discharge end of the duct. 5. Oven with low NOx emissions (2) according to claim 3, wherein the discharge end of the duct is shaped so that a radially outermost part of the duct extends further into the combustion chamber (8 ) than a radially innermost part of the duct for 45 divert the flow of combustion air discharged from the air hole (50) to the centrifuge (36). 6. Oven with low NOx emissions (2) according to claim 1, wherein the discharge ends of the Air holes (50) extend between approximately 25% and 50% of the distance between the oven wall (4) and one end downstream of the centrifuge (36). 7. Oven with low NOx emissions (2) according to claim 1, wherein the end downstream of the 50 centrifuge (36) is located inside the combustion chamber (8) at a substantial distance of 5 10 fifteen twenty 25 30 35 40 Four. Five fifty the oven wall (4), at least six separate, elongated air holes (50) arranged substantially equidistant around the tube (24) to flow combustion air into the combustion chamber (8), each air hole (50) having an end of discharge downstream that an intermediate distance from the kiln wall (4) that is less than the substantial distance is separated, a wall element arranged in cavities (64) proximal to the upstream ends thereof to prevent combustion air from flowing between adjacent air holes (50), the first plurality of fuel gas discharge pins (40) disposed around a periphery of the centrifuge (36) and the discharge holes (46) extend at least the substantial distance into the combustion chamber (8), Y The second fuel gas discharge tap (66) has a fuel gas discharge hole (68) to flow fuel gas into the combustion chamber (8) which is separated from the oven wall (4) a distance that is less than the intermediate distance. Oven with low NOx emissions (2) according to claim 1 or 7, wherein the burner with low NOx emissions (10) with a longitudinal axis adapted to generate a flame (84) in the combustion chamber (8) for generate furnace gases in the chamber (8) that are discharged as chimney gases after a furnace gas treatment, a combustion air source and a fuel gas source (12) to generate the flame (84), a combustion air duct for flowing combustion air from the source through the centrifuge (36) into the combustion chamber (8), the plurality of air holes (50) extend circumferentially equidistant apart from each other to define cavities (64), the discharge ends of the air holes (50) being upstream of the centrifuge (36), plates between adjacent pairs of air holes (50) that prevent combustion air from flowing from the combustion air source through the cavities (64), the first set of elongated fuel taps (40) extending from the fuel source (12) beyond the furnace wall (4) open into the combustion chamber (8) and the discharge holes of fuel gas (46) are separated from the oven wall (4) at least as much as the downstream end of the centrifuge (36) to discharge fuel gas into the combustion chamber (8) and mix the fuel gas with air combustion centrifuge (36), The fuel gas discharge port (68) of each second fuel gas tap (66) is arranged inside the combustion chamber (8), so that the fuel gas discharged by the second spigots (66) is mixed with furnace gas that recirculates in the combustion chamber (8) towards the furnace wall (4) and into cavities to form a non-combustible mixture of fuel gas-furnace gas upstream of the downstream ends of the air holes (50), the non-combustible mixture being further mixed with combustion air from the discharge ends of the air holes (50) upstream of the centrifuge (36) for subsequent ignition by means of the flame (84) in the chamber of combustion (8) substantially downstream of the centrifuge (36), and a fuel gas discharge regulator (82) operatively coupled with the fuel gas source (12) and the fuel gas spouts (40, 66, 72) to direct relatively more fuel gas through the second gas spindles fuel (66) than through the first fuel gas spikes (40). Oven with low NOx emissions (2) according to claim 8, wherein the spaces, the first fuel gas spouts (40), the centrifuge (36) and the combustion air duct are not obstructed in a radial direction in relationship with the shaft so that the recirculating combustible gas in the combustion chamber (8) can be freely flowed into the spaces and into the vicinity of the first fuel gas taps (40), the centrifuge (36) and the duct of combustion air to facilitate mixing of the combustible gas, combustion air and the recirculation furnace gas upstream of the downstream end of the centrifuge (36). Oven with low NOx emissions (2) according to claim 9, which includes a third fuel gas tap (72) disposed inside each air hole (50) and having a fuel gas discharge hole (76) located upstream of the discharge end of the air hole (50) to carry combustible gas into the combustion air flowing through the air hole (50) and to form a mixture there 10 fifteen twenty 25 30 35 40 Four. Five of combustible gas and combustion air. 11. Oven with low NOx emissions (2) according to claim 10, wherein the regulator (82) directs relatively less fuel gas to the third fuel gas spouts (72) than to the second fuel gas spigots (66) . 12. Oven with low NOx emissions (2) according to claim 8, wherein the discharge ends of the air holes (50) are inclined so that a radially outermost part of each air hole (50) is extends further into the combustion chamber (8) than a radially innermost end of the air hole (50) to thereby divert combustion air from the air holes (50) to the centrifuge (36). 13. Oven with low NOx emissions (2) according to claim 8, wherein the oven (2) includes a multiplicity of heat exchange tubing arranged inside the combustion chamber (8), and wherein The recirculation furnace gases come into contact with the heat exchange tubes and are cooled by the heat exchange tubes before the recirculation furnace gases are mixed with combustion air. 14. Method for lowering the NOx emissions of a furnace (2) that has an oven wall (4), a combustion chamber (8) inside the wall (4), a burner (10) that is extends into the combustion chamber (8) generating a flame (84) from combustion air and chimney gas discharged by the burner (10) in the combustion chamber (8), and a centrifuge (36) located on a longitudinal axis of the burner (10), the method comprising: - place the centrifuge (36) in the combustion chamber (8) so that the centrifuge (36) is located at a substantial distance from the oven wall (4), - directing a first combustion air flow through the centrifuge (36) and discharge the combustion air from one end downstream of the centrifuge (36) into the combustion chamber (8), - mixing downstream of one end downstream of the centrifuge (36) a first flow of combustible gas with the first flow of combustion air and igniting a resulting mixture thereof to generate the flame (84) in the combustion chamber (8 ), - arranging a plurality of spaced combustion air streams, separated around the first combustion air flow and discharging the combustion air streams into the combustion chamber (8), - forming cavities substantially free of combustion air (64) between adjacent streams of combustion air upstream from where the combustion air streams are discharged into the combustion chamber (8), - separately flowing a second combustible gas into the cavities (64) in a direction towards the centrifuge (36) providing a second fuel gas tap (66) arranged between the adjacent pair of air holes (50) , said second fuel gas tap (66) being arranged in relation to the axis proximal to the radially outermost parts of the air holes (50), - recirculating furnace gases from the combustion chamber (8) into the cavities (64), by flowing from the cavities (64) the recirculated furnace gas to the centrifuge (36), and dragging the second flow of combustible gas into the combustion air recirculated in the cavities (64) to form a fuel gas-oven gas mixture, - mixing the fuel gas-furnace gas mixture with the combustion air streams upstream of the centrifuge (36) to form a fuel mixture of fuel gas / furnace gas / combustion air flowing in a downstream direction beyond of the centrifuge (36), and - ignite the fuel gas / combustion gas / combustion air mixture with the flame (84) generated by the centrifuge (36). 15. Method according to claim 14, which includes dragging a third flow of combustible gas into combustion air streams before combustion air streams are mixed with the fuel gas-furnace gas mixture, the third flow being of combustible gas larger than the first flow of combustible gas and smaller than the second flow of combustible gas.